Of course with no tongue weight, the control of the trailer goes out to lunch, with the trailer jumping left to right and possibly taking the load and the towing vehicle with it. Too much and you can damage your hitch.
I have actually seen somewhere an illustration on how to determine the tongue weight on your trailer using a bathroom scale. Of course, the tongue weight alone would kill most bathroom scales, even the Canadian ones which are made to measure the majority of Canadian women. (Hopefully Karmen doesn't read this thread, and it cetrtainly does not apply to her). Seems to me they used an extension under the hitch of about 3 feet, and made some multiplication of the results.
http://www.curtmfg.com/index.cfm?eve...ntpieceid=1347
The link above shows you how they did it.
Once you have set it up on your specific trailer/carrier combination, you can make marks to repeat it each time afterwards.
Personally, I measure the hitch height before I put the carrier onto the trailer, and then move the carrier until I get about 3" less height.
I put my ferret on my trailer coming home from an airshow and miscalculated. The thing was just about in the other lane. How I managed to stop, I don't know, but I near had to change my shorts afterwards.